The present invention relates to the field of current collectors, electrodes and lead-acid batteries. More particularly, the present invention relates to current collectors composed of a lead or lead alloy substrate and a tin cladding applied to the substrate surface and batteries utilizing such current collectors, such batteries being characterized by both high cycle life and long shelf life. The present invention further relates to methods for manufacturing batteries utilizing this type of current collector.
Despite considerable research into the development of improved electrochemical storage devices, the lead-acid battery remains a predominant device for delivering electrical current in many electrical operations. A conventional lead-acid battery such as the valve-regulated lead-acid (VRLA) battery is comprised of a plurality of cells. Each cell typically includes a set of interleaved monopolar positive and negative electrodes or xe2x80x9cplates.xe2x80x9d The electrodes typically are composed of a lead or lead-alloy current collector and an electrochemically active paste which is coated onto a surface of the current collector. The current collector typically is in the form of a grid but may also be in other forms such as a solid foil or film. The paste on the positive electrode plate contains lead dioxide when charged and is called the positive active material; the negative electrode contains a negative active material, typically sponge lead. Electrodes of opposite polarity are separated one from the other by a porous electrically insulating separator material such as a glass microfiber mat. The cell is completed by adding an acid electrolyte between the positive and negative electrodes and enclosing the entire assembly within a suitable case. A charging process activates the cell.
A major goal in the field of lead-acid batteries is to develop batteries having increased cycle life and longer shelf life. Cycle life is defined as the number of discharging and recharging cycles a battery can sustain while still delivering a certain level of electricity. Cycle life is dependent upon a number of factors including testing conditions and cell construction. With regard to testing parameters, for instance, a cell which achieves 80% of its initial amp-hour rating after 500 cycles but delivers only 50% of its initial amp-hour rating after 1,000 cycles will have two different cycle life values, depending upon whether the cell is rated at 80% or 50% of initial capacity. A related parameter, xe2x80x9ctotal useable capacity,xe2x80x9d refers to the number of cycles achieved during the cell""s life multiplied by the amp-hours delivered during each cycle. It is equivalent to the area under a curve in which discharge capacity (in amp-hours) is plotted against cycle number and is also a measure of the useful work a cell can provide.
Shelf life simply refers to the usable life of a battery when it is not in use. The shelf life of batteries is affected by a process called xe2x80x9cself-discharge,xe2x80x9d i.e., chemical reactions within the cell which cause the consumption of electrolyte, even when the cell is not exposed to an external load. The consumption of electrolyte through self-discharge decreases discharge capacity because the discharge capacity of a cell is proportional to the specific gravity, or concentration, of electrolyte within the cell. Self-discharge not only reduces storage time and discharge capacity but also results in voltage decay, or a decrease in open circuit voltage.
Cycle life and shelf life are dependent in large measure on the chemistry which occurs at the interface between the current collector of the positive electrode and the electrochemically active paste. This interface is referred to as the xe2x80x9ccorrosion layerxe2x80x9d or xe2x80x9cpassivation layerxe2x80x9d depending on the conductivity of the layer. While all of the chemistry that takes place at this interface is not fully understood, battery technologists currently believe that a conductive corrosion layer (which may be a semi-conducting layer) is necessary to obtain long cycle life in lead-acid batteries. However, with certain lead and lead-alloy grids or foils, in particular pure lead, lead-calcium and lead-low tin compositions, a passivation layer (i.e., a non-conducting layer) can form. Passivation layers are composed primarily of lead oxide (PbO). The lead oxide acts as an electrical insulator and can reduce conductivity such that current cannot pass from the active material through the layer without a significant voltage loss. Thus, whether a conductive or passivation layer exists at the current collector/paste interface can dramatically impact the electrochemical properties of a cell.
In particular, the formation of a conductive corrosion layer, achieved at least in part through appropriate selection of current collector composition, beneficially results in a cell having a long cycle life. However, the drawback to a corrosion layer is that the cell generally has reduced shelf life due to the ongoing corrosion or oxidation of the lead or lead alloy current collector which consumes needed electrolyte. In contrast, current collectors whose composition tends to create passivation layers have excellent shelf life but relatively poor cycle life and recovery from deep discharge and stand. The extended shelf life is a consequence of the passivation of the corrosion layer which protects the current collector from corrosion and self-discharge and thus voltage decay; yet, as noted above, the passivation process also acts to inhibit current flow during charging, thereby reducing cycle life. Thus, cycle life and shelf life are inversely related with regard to the effect of the conductive/passivation layer. A conductive corrosion layer enhances cycle life but reduces shelf life; a passivation layer, in contrast, negatively affects cycle life but increases shelf life. Consequently, cell design involves choosing materials with the realization that a composition which enhances cycle life typically involves a tradeoff wherein shelf life is sacrificed and vice versa.
One approach to optimizing cycle life and shelf life has been to utilize lead-tin alloy current collectors in place of traditional pure lead current collectors. It has been found that the inclusion of small percentages of tin in the grid reduces the formation of a passivation layer, thereby enhancing the cycle life of a cell. It is thought that a relatively high tin content results in the tin being corroded, presumably to soluble tin(II) or insoluble SnO2. The corroded tin compounds are incorporated into the passivation layer where the tin compounds act as a conductor to ameliorate the insulative effects of the passivation layer, thereby enhancing conductivity and current flow between the current collector and the positive active material.
Several patents describe current collectors in which a lead-tin alloy film is superimposed on a lead or lead alloy substrate. United States patents describing this approach include U.S. Pat. Nos. 4,107,407 to Koch, 4,939,051 and 4,805,277 to Yasuda et al., and 4,761,356 to Kobayashi et al. Unlike the present invention, these patents do not describe the use of a pure tin cladding, nor do they describe current collectors wherein the tin is distributed only at the very outer surfaces of the current collectors and especially wherein the tin is nonhomogeneously distributed at the surface such that there are particles or regions of tin interspersed among high lead regions at the current collector surface.
In U.S. Pat. No. 5,024,908 (the xe2x80x9c908 Patentxe2x80x9d) to Terada et al., a tin-coated substrate is prepared for use in a lead acid cell. However, the 908 Patent teaches away from the current collectors of the present invention in which tin is clad to the outer surface of a substrate by stating that there are problems associated with using current collectors which are tin plated. In particular, the 908 Patent claims that during charging and formation the tin plating can disintegrate to create tin particles that can form deposits at the cathodic plates; these deposits, in turn, can cause a short circuit in the cell. Hence, the 908 Patent teaches that the tin-coated substrate of the current collector should be heated at a temperature of at least 170xc2x0 C. in order to effectively diffuse the tin into the substrate matrix. This contrasts sharply with the present invention wherein the tin has been found to provide significant enhancement in cell performance if located at the surface of the current collector.
Because existing electrochemical cells fail to fully optimize both cycle life and shelf life, there remains a need for current collectors, batteries and methods for making these devices which can significantly increase the cycle life of a lead-acid battery while at the same time maintaining long shelf life, or alternatively, which optimize shelf life without compromising cycle life.
The present invention addresses and fulfills the need identified above. In particular, the present invention generally provides improved current collector compositions, batteries utilizing such current collectors and methods for manufacturing such batteries, wherein the batteries have both improved shelf life and cycling performance relative to other prior art batteries.
More specifically, the present invention provides a current collector which comprises a tin-clad lead or lead alloy substrate and lead-acid batteries based upon such current collectors. Preferably, the cladding is substantially pure tin and comprises less than 4% of the current collector by weight (all tin cladding weight percentages listed herein are expressed relative to the combined weight of the lead or lead alloy substrate and the tin cladding which comprise the current collector).
The tin cladding enhances shelf life and cycle life performance. The current inventors have found that the key to simultaneously achieving long shelf life and cycle life is to have a semiconducting layer on the outer surface of the current collector. The current collectors provided for in the current invention satisfies this important criterion. Batteries utilizing the tin-clad substrates described herein have superior cycle life and shelf life performance relative to prior art cells. The particular enhancement observed is a function of the tin concentration at the current collector surface. In general, a higher concentration of tin results in particularly high cycle life. At lower tin cladding levels, shelf life performance can be optimized.
The current collectors provided in the current invention have the further advantage that they can be manufactured using standard processing techniques and require less materials, especially tin, thereby reducing manufacturing costs. The current collectors can also be used with all types of electrochemically active pastes and paste additives to achieve similar performance enhancement. The tin-cladding can be applied to substantially pure lead or to a lead alloy substrate and yield a current collector capable of delivering improved performance characteristics when incorporated in a battery. Further, when such current collectors are used in batteries, the tin cladding also maintains the deep discharge recovery of the cell, while preserving the sealed, valve-regulated characteristics of the battery.
The current collector provided by the present invention comprises a lead or lead alloy substrate and a tin cladding located at the surface of the substrate. Preferably, the current collector includes a thin lead-based foil or film having two primary faces to which a very thin layer of tin cladding is applied. The lead substrate may optionally be either substantially pure lead or a lead-alloy. If prepared from substantially pure lead, the lead substrate is preferably 99.9% lead, and most preferably 99.99% lead (all purity values listed herein are on a weight percent basis). If the lead substrate is a lead alloy, the lead alloy is preferably a lead-tin, lead-antimony, lead-calcium-tin or lead-calcium-antimony alloy, although other alloy compositions could be used as well. The current collector may include a variety of shapes or forms, including for example a grid or a substantially non-perforated foil or film. The current collector is typically less than 0.07 inches thick; however, preferably the current collector is less than 0.007 inches thick, and most preferably is approximately 0.002 to 0.003 inches thick.
The tin cladding clad to the lead substrate does not form a continuous layer over the surface of the lead substrate. Hence, the lead or lead alloy substrate breaks through the tin cladding, leaving only small particles or regions (depending on the tin concentration) of concentrated tin on the electrode surface. Thus, the current collector surface has pockets of high tin density separated by regions which have no tin.
The tin cladding is preferably substantially pure tin, although this is not required. If substantially pure tin is used, the tin preferably has a purity of at least 99.9%. The concentration of the tin cladding relative to the combined weight of the lead substrate and cladding is preferably less than 4%, more preferably between 0.001% and 0.5%, and most preferably between 0.001% and 0.05%. The thickness of the tin cladding preferably ranges from 0.01 to 100 microns, and most preferably between 0.5 and 2 microns.
The present invention further provides electrochemical cells or batteries utilizing the current collectors described above. In particular, the cells are characterized by positive and negative electrodes which are composed of the current collectors described above and an electrochemically active paste or active material Although preferably both the positive and negative electrodes or plates include the current collectors described herein, it is possible to make cells in which only the positive electrode includes such current collectors. The positive and negative electrodes are arranged in an alternating fashion and are separated by an electrically insulating separator, the combination of a positive electrode, separator and negative electrode defining a unit cell. A unit cell or collection of unit cells encapsulated in a container having an electrolyte yields a lead-acid battery having both superior cycle life and shelf life characteristics.
The positive and negative electrodes can be of a variety of sizes and thicknesses. The preferred thickness of the positive and negative electrodes (current collector plus paste on both sides of the current collector) is preferably less than 0.07 inches and most preferably between 0.009 and 0.015 inches.
The electrochemically active paste can be an unsulfated paste but preferably is a sulfated paste. More preferably, the paste is a sulfated paste containing a tin compound. The tin compound can include tin sulfate, SnO, metallic tin, tin (II) salts and tin (IV) salts. In an especially preferred embodiment, the paste is a sulfated paste which includes tin sulfate, the tin sulfate concentration preferably being between approximately 0.1 and 2.0 percent of the sulfated paste by weight.
Additionally, the present invention further provides a method of manufacturing batteries wherein at least the positive electrode includes a current collector as described herein. The method involves preparing a positive current collector by cladding an outer surface of a lead or lead alloy substrate with a tin cladding and preparing a negative current collector, which preferably also involves cladding a foil or film with tin. A separator is positioned between the positive and negative current collectors. The combination of a positive and negative current collector and the separator imposed therebetween, or alternatively a plurality of positive and negative current collectors and their associated separators, are then encapsulated with an electrolyte in a container.